There is no radiation countermeasure approved by the FDA that meets the criterion of a radiomitigator - an agent, which mitigates the acute radiation syndrome when administered after the exposure. The central hypothesis of the present is that selective activation of the lysophosphatidic acid receptor subtype 2 (LPA2) provides protection against radiation injury even if applied after radiation exposure. The overall objective of this project is to develop a second generation of small molecule radiomitigator countermeasures targeting the LPA2 receptor. This objective is based on over a decade of research, in which we have identified and validated lysophospholipid receptors as molecular targets that rescue apoptotically condemned cells in vitro and decrease mortality in vivo after radiation injury. Using in silico drug discovey based on pharmacophore models of the LPA G protein-coupled receptors, we have identified two small non-lipid drug-like selective LPA2 agonists - GRI977143 and NSC 12404. In preliminary experiments we have established that GRI977143 protects and rescues cultured cell in vitro from direct 3-radiation-induced cell death. GRI977143 also prevents radiation-induced bystander apoptosis elicited by the transfer of spent medium of irradiated U937 human histiocytic cell line cultures to non-irradiated IEC-6 intestinal epithelial cell or U937 cell cultres. When applied 24 h after ~7 Gy irradiation in mice, GRI977143 decreased mortality. However, the potency of these two compounds is suboptimal and more potent analogs are needed. To build on these preliminary data, here we propose to identify analogs of these molecular scaffolds and develop novel non-lipid drug-like agonists of the LPA GPCR with improved pharmacological and pharmaceutical properties for the mitigation of the hematopoietic and the gastrointestinal acute radiation syndromes. We will combine our in silico drug discovery expertise, high-throughput experimental screening platforms, medicinal chemistry and pharmaceutics capabilities to develop a new and more effective radiomitigator countermeasure. We propose to: 1. Implement computationally guided drug discovery for LPA2 agonists. 2. Characterize the selected nonlipid agonists for radiomitigating action in vitro. 3. Evaluate the radiomitigating efficacy of selected nonlipid agonists in vivo. Radiomitigators are needed in the Strategic National Stockpile for the protection of our military, first responders and the public at large. The VA System is the likeliest site where many of the casualties of a nuclear accident or terrorist attack would be treated underlining the significance of the proposed research.